Governing system for reheat steam turbine powerplant



P. G. IPSEN ETAL Filed Aug. 22, 1958 Sept. 8, 1959 GovERNING SYSTEM FORREHEAT STEAM TURBINE POWERPLANT .n.me

Unite States Patent GOVERNING SYSTEM FOR REHEAT STEAM TURBINE PGWERPLANTPeter G. Ipsen, Schenectady, and Gordon W. Clark,

Burnt Hills, N.Y., assignors to General Electric Company, a vcorporationof New York Application August 22, 1958, Serial No. 756,607

2 Claims. (Cl. 60-73) This invention relates to steam turbinepowerplants, particularly to very large capacity compound turbinesoperating with steam at an initial pressure above the critical pressure,meaning at least 3206 p.s.i.a., and is particularly applicable to suchpowerplants having two heaters for reheating the steam after partialexpansion in certain of the turbine sections.

The present invention is a further improvement of the reheat turbinegoverning system of U.S. Patent 2,811,837, issued November 5, 1957 inthe name of M. A. Eggenberger, and assigned to the same assignee as thepresent application. As noted in considerable detail in that patent, aproblem arises from the fact that super-critical pressure steamgenerators of the forced circulation or once-throug type must beprovided with means for recireulating steam during the starting cycle,without passing it through the turbines, until the steam generatingcycle reaches appropriate temperature and pressure conditions. This isaccomplished by providing variously arranged valves for bypassing thesteam to the condenser and back to the steam generator withouttraversing any of the turbines. Such steam generators require a minimumflow rate of about 30% of the maximum flow rate and approximately 30% ofrated pressures in the -reheaters in order to bring the steam supplytemperature up to proper values for staiting the turbine; and, uponsudden decrease in load, the flow rate must be kept at least at a valueof 30% of the maximum ilow rate if damage to the boiler is to beavoided, since such a minimum ilow rate is necessary in order to preventoverheating of the various tubes in the steam generator.

With a steam generator of the type described, it is of the utmostimportance to exercise accurate control of the steam supply to therespective turbine sections during the starting cycle; and propersynchronization of the valve actuators is found to be difficult wherethe various valves required are widely separated. Furthermore, it isimportant that the steam supplied initially during the starting cycle beaccurately metered to the respective turbine sections, in suicientquantities as to provide the power required to bring the turbine rotorup to rated speed, and also to remove the heat generated by turbulencedue to the fact that normal design flow conditions are not produced bythe low turbine flow and 30% of rated reheater pressures. Without suchventilation the friction heat resulting from this turbulence wouldresult in overheating of certain of the high pressure turbine sections.

Accordingly, an object of the present invention is to provide improvedvalve gear for a double reheat compound turbine with special means foradmitting high pressure steam to produce no-load power, and ventilatethe turbine sections, during the starting cycle.

A further object is to provide reheat turbine valve gear of the typedescribed actuated by mechanism permitting very accurate operation inproper sequence of the starting valves, in order to appropriately meterhigh pressure steam to the respective turbine sections.

2,902,831 Patented Sept. 8, 1959 Another object is to provide steamturbine governing mechanism of the type described employing valves andpipes of minimum size.

Still another object is to provide starting valve mechanism for acompound reheat turbine using high pressure steam, so as to use aminimum quantity of steam for performing the starting and Ventilatingfunctions.

Other objects and advantages will become apparent from the followingdescription taken in connection with the accompanying drawing, in whichthe single figure is a diagrammatic representation of a multi-sectiondouble reheat steam turbine powerplant supplied with supercriticalpressure steam from a boiler of the forced circulation type and havingspecial starting valve means arranged in accordance with the invention.

Generally sta-ted, the invention is practiced by provid- K ing a doublereheat compound steam turbine with separate starting valves foradmitting carefully controlled quantities of steam from the highpressure steam supply directly to each of the high pressure andintermediate pressure turbine sections, so as to bring the turbine rotorup to normal rated speed while providing sucient steam ilow to removethe heat generated by turbulence in the turbines due to the fact thatthe flow passages are not operating at full design conditions, withresulting ineiciency which shows up as friction heat.

Referring now to the drawing, the invention is illustrated as applied toa compound double reheat steam turbine powerplant having a very highpressure or super pressure turbine section 3, a high pressure turbinesection 4, two intermediate pressure turbine sections 5, 6, and two lowpressure sections 7, 8. The turbine elements 3, 5, 7 comprise the rstturbine unit 1; and elements 4, 6, 8 comprise the second turbine unit 2.As will be appreciated by those acquainted with such powerplants, theturbine units 1 and 2 may have mechanically independent rotors,connected to separate generators, not shown. The drawing shows inconsiderable detail the piping and valve mechanism for connecting theseturbine sections with the steam generator, and includes a complex bypassarrangement for starting the steam generator while bypassing steamthrough the reheaters and to the condenser without going through theturbines. The full mechanical details of this bypassing arrangement arenot necessary to an understanding of the present invention, but theshowing in the drawing taken with the general outline of the operationbelow will be helpful in understanding the advantages of the startingvalve mechanism to which this invention particularly relates.

This starting valve arrangement comprises the pilot valve member 10 ofthe main control valve 9, and the special starting Valves 11 and 12. Aswill be seen more particularly in the description of the operationbelow, these valves must be operated in a precise sequence relative tothe high pressure intercept valve 13, and the two intermediate pressureintercept Valves 14 and 15. To this end, the valves 9, 11, 14 and 12 maybe actuated by a number of cams disposed "on a common cam-shaft 16carrying a pinion 17 adapted to be positioned by a rack 18 carried on anactuating rod 19 pivoted to the main governor lever 20a The interceptvalves 13, 15V are likewise actuated by cams 21, 22 carried on a commoncamshaft 23 actuated by pinion 24 and rack 25 carried on the rod 26pivoted to the left-hand end of the pre-emergency governor lever 27, Themain governor 28 is at all times free to act, upon increase in'speedabove the value for which it is set to lower the link 30, and thusrotate the shaft 31 so as to pull downwardly on the lost-motion 23 tomove the valves 13, 15 towardvthe closed condition.

Likewise, the pre-emergency governor 3,4 is always free to pull down onlever 27, irrespective of the condition of governor 28, to close theintercept valves 13, 15 if the speed for which governor 34 is set isexceeded.

In normal operation, the main governor 28 positions the lever 20 so asto actuate the cam-shaft 16 to position the main control valve 37 so asto hold speed or load at the value determined by the setting of thesynchronizing device 29. If it is desired to establish a maximum loadbeyond which the governing mechanism cannot admit more steam to theturbine, the adjustable load limit abutment 35 may be positioned towardthe right-hand end of lever 20 so that the lever cannot move upwardlymore than an amount corresponding to this preselected maximum load. Instarting the powerplant, this load limit abutment 35 performs anadditional function noted hereinafter.

It will be apparent from the drawing that the actuating rod 36 of themain control valve gear 9 acts first to raise the pilot valve disk so asto admit a small quantity of steam through the port provided in the mainvalve disk member 37, in a manner which will be apparent to thoseacquainted with turbine valves. After moving a preselected distance, thevalve disk 10 engages the top of the main valve disk 37 and causes it torise from its seat.

The starting valve /11 is connected to supply appropriate quantities ofsteam to the inlet end of the intermediate pressure turbine 5 by way ofconduit 38 and a How-limiting orifice 39, also by way of conduit 38 tothe inlet end of the intermediate pressure turbine 6, past aflow-limiting orifice 40.

The starting valve 12 is connected to supply carefully meteredquantities of steam through conduit 41, past a flow-limiting orifice 78, to the inlet end of the high pressure turbine section 4.

As will be appreciated by those acquainted with compound reheat typeturbine powerplants, the intercept valve 14 is an emergency stop valvearranged to shut off the supply of motive fluid to the intermediatepressure turbine 5 from the low pressure reheater 42. Similarly, thehigh pressure intercept valve 13 serves to shut ofi the flow of steamfrom the high pressure reheater 43 to the inlet end of the high pressureturbine section 4. Finally, the intercept valve serves to stop the ow ofsteam from the low pressure reheater 42 to the inlet end of theintermediate pressure turbine 6.

Numerous other details of the system shown will be I readily apparentfrom the drawing to those acquainted with compound steam turbinepowerplants. The steamV generator comprises the boiler 44 having aconventional boiler feed pump Y45 and feedwater heater system 46, thedetails of which are not important here. From the boiler, the steampasses to a superheater section 47 and after passing through certain ofthe turbine sections :it is returned to the high pressure reheater 43and, after further expansion, to the low pressure reheater 42. The mainstop valve 48, disposed immediately upstream from the main control valvegear 9, is arranged to be closed quickly by an overspeed governor 49 inthe event turbine speed rises to a preselected maximum value. The reheatstop valves 72, 73 and 74 are also arrangedV to be closed quickly bysuitable similar overspeed governors,V not shown. The speed governor 34is known as a pre-emergency governor, and acts to begin to close theintercept valves 13,v 15 before the turbine rotor reaches the speed atwhich the main stop valve 48 would be tripped.

Other details of the piping and the special bypass valves required inthe operation of the system will be apparent from the outline of theintegrated operation given below, In describing the operation of thecomplete plant, it is to be noted that three principal operatingconditions are represented by different flow path arrows, as indicatedby the legend on the drawing. Solid arrows represent steam flow innormal operation. Dash-dot arrows represent the flow during the startingcycle of the steam generator, before motive fluid is admitted to theturbines. Dotted arrows 4 illustrate the starting cycle of the turbines.Dash lines indicate condition-sensing signals; and a pressure signal isrepresented by an encircled P, and a temperature signal by an encircledT.

The complete operating cycle of the plant is as follows.

Assume iirst that the steam generator and turbine are cold. The boilerfeed pump, driven by a suitable electric motor (not shown) operates tocirculate cold water through the boiler 44, superheater 47, then throughthe bypass valve 50, conduit 51, water separator 52, thence throughconduit 53 direct to the condenser hot-well 54. When approximately 30%of rated iiow is thus established, the boiler is ired and thetemperature of the water begins to rise. The boiler feed pump is sooperated that the water pressure rises to the full rated pressure, whichmay for instance be 3500 pounds per square inch, gage. Thus the waterremains in the liquid phase with no evaporation taking place. Thesuper-pressure bypass valve 50 is so controlled by the temperature andpressure sensing connections shown in the drawing that this 3500 poundsper square inch pressure is maintained in the superheater and boiler.However, as the water temperature rises, it eventually acquires enoughthermal energy that a certain amount of it flashes into steam as itpasses through the valve 50. The steam is separated from that portionwhich remains liquid in the separator 52. As before, the water isreturned by conduit 53 to the condenser hot-well 54. The vapor passesfrom the upper portion of water separator 52 by way of conduit 55through conduit 56, the high pressure reheater 43 thence through conduit57 to the high pressure bypass valve 58, and through conduit 59 to thelow pressure reheater 42, and by way of conduit 60 and low pressurebypass valve 61 to the condenser 62.

As the heat input to the boiler 44 increases, the temperature of the uidrises to the point Where all the fluid passing through thesuper-pressure bypass valve 5) is steam, which temperature may be on theorder of 875 F. This is a suiciently high temperature that it woulddamage the piping and condenser tubes. Therefore, water is supplied tothe desuperheating water spray chambers 63, 64, and 65. Thesedesuperheaters are of course provided with automatic temperature controldevices (not shown) so the water injected ymaintains the steam exittemperature at safe values.

During this process, the high pressure bypass valve 58 and the lowpressure bypass valve 61 are adjusted manually so as to gradually raisethe pressures maintained in the high pressure reheater 43 and the lowpressure ref heater 42 to values about 30% of the rated values; that is,on the order of 350 p.s.i.g. and p.s.i.g., respectively.

Thus the steam generating system has arrived at operating conditions atwhich it is appropriate to begin admitting steam to kthe turbines. lnorder to warm up the super-pressure turbine 3 and the high pressureturbine 4 (in order to limit the thermal shock which occurs when steamat the temperature now produced by the steam generator is suddenlyadmitted to the cold turbines), a small amount of warming steam isadmitted by manually opening the valve 66. This admits steam directlyfrom the high pressure reheater 43 to the discharge end of turbine 3.Similarly, valve 67 is opened to admit steam from conduit 81, which isshown connected to conduit 60. This admits steam from the low pressurereheater 42 through conduits 60, 81, 69to the discharge end of the highpressure turbine 4. This steam brings the turbine casings 3 and 4 up toabout 30% of rated pressure and supplies enough heat to bring theseturbines to temperatures which may for instance be on the order of 420F. for the super-pressure turbine 3 and 330 F. for the high pressureturbine 4. Suitable check valves are provided as shown in the drawing toprevent reverse flow through the warming valves- 66, 67. Once thesuperpressure turbine 3 and the high pressure Iturbine 4 are brought upto pressure and appropriately warmed, the valves 66, 67 are manuallyclosed.

The boiler stop valve 70 is now opened manually, admitting steam to theconduit 71, which contains the' main stop valve 48, located immediatelyupstream from the main control valve gear 9. Now it is necessary tomanually open the main stop valve 48, the high pressure reheater stopvalve 72, and the low pressure reheater stop valves 73 and 74.

The adjustable load limit device 35 also serves as a manual startingdevice, and is now backed away from the right-hand end of lever 20,which follows the load limit abutment upwardly, by reason of the bias ofthe spring in governor 28. This causes rack 18 to rotate the cam shaft16 so that cams 75, 76, and 77 begin to open the pilot valve in the maincontrol valve 9 and the other two starting valves 11, 12, respectively.This admits steam at an initial pressure of 3500 p.s.i.g. into thesuperpressure turbine section 3, the intermediate pressure turbinesection 5, and the high pressure turbine section 4. Steam is alsoadmitted through conduit 38 to the intermediate pressure turbine section6. This ow is limited by suitable orices 39, 40 and 78 so as to maintaina high pressure in these conduits, permitting them to be of smallerphysical size than would be required if the steam were permitted toexpand freely therein.

The contours of the cams 75, 79, 77, are so selected as to cause thethree starting valves 10, 11, 12 to admit quantities of steam to theturbines 3, 5, 4, and 6 adequate to provide the no-load power to bringthe turbine up to normal rated speed, which may for instance be 3600r.p'.m. The quantity of steam thus provided is also so selected as toremove the heat generated by turbulence in the respective turbinesections, as noted above.

As the speed of the turbine rotor reaches the range in which thegovernor 28 becomes eiective, the synchronizing device 29 is actuated tocontrol the speed and appropriately bring it up to the desired ratedspeed.

Once rated speed is established, the generator driven by the `turbine(not shown) can be connected to the electrical network to which itdelivers power. The governor 28 will now maintain stationary in spacethe pivot point 80, and the synchronizing device 29 can be actuated tofurther open the starting valves 10, 1,1, 12. Any tendency of theturbine rotor to overspeed is 'prevented by the fact that the generatoris delivering power to a 60- cycle A.C. network to which other constantspeed turbines are also delivering power, and the electricalcharacterisitcs of the system keep the generator approximately at 3600R.P.M. irrespective of .additional opening of the starting valves.

This action continues until the cam-shaft 16 brings the starting valves10, 11, 12 to their fully open condition. At this point, cam 76 beginsto open the intermediate pressure intercept valve 14, cam 21 begins toopen the high pressure intercept valve 13; and cam 22 starts to open theintermediate pressure intercept valve 15. y IIhis causes steam to beadmitted from the high pressure reheater 43 through conduit 57 to valve13 and to the inlet of the high pressure turbine section 4. Also, steamis admitted from the low pressure reheater 42 through conduit 60 andconduit 81 to the intermediate pressure intercept valve 15 and to theinlet of intermediate pressure turbine 6. Steam is also admitted fromthe low pressure reheater 42 through conduit 60, conduit 68 to theintermediate pressure intercept valve 14, and to the inlet ofintermediate pressure turbine 5. At the same time as the interceptvalves `14, 13, and 15 begin to open, the valve actuating rod 36 beginsto lift the main governing valve disk 37.

Speed synchronization is necessary in order that the two units be lockedin step electrically prior to connecting to the electrical network.Synchronization is also desirable in order that the proper operatingsequence and timing, as accomplished by the valve cams, will produceoptimum temperature and pressure conditions. In order to' synchronizeunits 1 and 2, the generator rotors must be turning within approximately5% of rated speed or any predetermined traction of rated speed in orderto prevent serious rotational acceleration or overheating of the fieldsat the moment the held is applied. Speed synchronization between theunits shown is accomplished by means of speed setting screw 33. Sincethe superpressure steam being supplied by starting valve 12 to the highpressure unit 4 would normally be inadequate to bring -unit 2 to Withinapproximately 5% of its rated speed where the shaft-driven exciterscould safely provide the synchronizing excitation necessary to lock therotors electrically, additional steam must be supplied to unit 2. Uponunit 1 reaching rated speed, therefore, at no load, additional steam maybe supplied to unit 2 to bring it within lthe synchronizing range bymanipulating speed setting synchronizing screw 33 so as to cause highpressure intercept valve 13 and intermediatepressure intercept valve 15to open and overcome this deficiency. As soon as synchronization hasbeen accomplished, speed setting screw 3.3 may be returned to itsprevious position and the starting sequence again is in control of thegoverning system with its associated cams.

Synchronization may also be accomplished during the starting cycle atless than rated speed, for example at 50% rated speed, which isapproximately the minimum speed at which the shaft-driven exciters willbecome effective to supply proper excitation to the generator field.The' operation during synchronizing .is the same, it only beingnecessary irst to adjust synchronizing device 29 to operate unit 1 at50% speed and then to manipulate speed setting synchronizing screw 33 soas to bring unit 2 within approximately 5% of half of its rated speed.When' this is done, the shaft-driven exciters are providingsulhcient'voltage to allow units 1 and 2 to be interconnectedelectrically and to allow the starting process to proceed with thegenerator rotors of units 1 and 2 turnning in phase.

Further actuation of the synchronizing device 29 begins to close thestarting valves 11 and 12. The shape of the respective cams are sodesigned that the intercept valves 14, 13, and 15 admit steam to therespective turbine sections at such rates as to provide the requiredpower for synchronizing, and adequate ow of steam yfor ventilation toprevent overheating.

The turbine is now producing .a significant power output and furtheractuation of the synchronizing device 29 increases this load. As theload increases, the starting valves 1I, 12 progressively close, and themain valve 9 and intercept valves 13, 15, and 14 open. By the time thepower output reaches approximately 30% of rated load, the interceptvalves 14, 13, 15 are wide open and the starting valves 11, 12 arecompletely closed. This is the normal operating condition.

During this starting process, the super-pressure bypass valve 50 hasbeen actingautomatically to hold the discharge pressure from superheater47 at the rated pressure of 3500 pounds per square inch absolute and toby-pass through conduit 51 that portion of the flow from the steamgenerator which cannot pass through the starting valves 11, 12, and 10.By the time 30% load is reached, the super-pressure bypass valve 50 iscompletely closed, and all the steam 4iiow is going through the maincontrol valve 9. Similarly, the high pressure bypass valve 58 and thelow pressure bypass valve 61 have been progressively closing, .and reachfully closed position at the 30% load condition. All the Iboiler ow isnow passing through the main inlet valve 9 to turbine 3, and throughintercept valves 14, 13 and 15 to the turbines 5, 4, and 6,respectively.

The plant is now in normal operation and the starting valves 11, 12 areinactive. The governor 28 now operates to position the main controlValve 9 to determine the power output, the speed continuing to be heldcon- 7 stant by the electrical interconnection with the power network.

In the event the load on'the turbine drops below the condition, theoperation is somewhat the inverse. That is, the cam-shafts 16, 23 beginto close the intercept valves 14, 13, i5, and the starting valves 11, 12again begin to open so as to maintain the required Ventilating flowthrough the respective turbine sections. The manner in which this iseffected will be obvious from the above description of the startingprocess.

If the load decreases suiiiciently, the intercept valves will becompletely closed and all the boiler flow will again be owing throughthe starting valves 10, l1, i2. Upon further decrease, the startingvalves progressively close and an increasing quantity of steam isbypassed by automatic actuation of the valves 50, 61, and 58. When allthe ow is traversing these bypasses, with the starting valves completelyclosed, the steam generator is in condition to be shut down.

Thus it will be seen that the starting valve mechanism to which theinvention particularly relates aiords accurately controlled and reliablemeans for metering carefully predetermined quantities of steam to therespective turbine sections so as to supply desired amounts of powerduring the starting cycle while maintaining tlow rates adequate toprovide the ventilation required to carry away the heat generated byfluid friction produced by the substandard flow conditions in therespective sections. It will that the starting valves'may be physicallygrouped close together and actuated by common mechanism, capable ofeffecting very accurate synchronization of the operation of therespective valves. It should, however, be understood that the actuatingmechanism shown in the drawing is purely diagrammatic, and manyequivalent electrical, hydraulic, or pneumatic servo-mechanisms might beemployed to actuate the respective valves in the sequence prescribedherein. By using high pressure steam taken be apparent from the abovedescription of the structure '30 directly from the main inlet conduit82, the starting valves y and their related conduits may be madecomparatively small. This is advantageous because the compactarrangement of the valves makes it important that they all be as smallas possible because of the physical limitations of the space availablefor the interconnecting piping..

around the valve chest. Also, the physical size of the valves themselvesbecomes an important factor when so many valves are closely grouped.

Numerous modifications and substitutions of equivalents will be apparentto those skilled in the art, and it is f l.

y'lower pressure turbine section connected to receive motive fluid fromthe reheater, intercept valve means controlling the ow ofvsteam from thereheater to the lower pressure turbine, and a condenser receiving spentmotive fluid from the lowest pressure turbine, the combination of meansincluding pressure regulating valve means for automatically bypassingfluid from the steam generating means to the condenser during thestarting cycle, starting valve means connected to admit limitedquantities of motive uid from the steam generating means at said inlettemperature and pressure to the rst turbine section in quantitiessuiicient to supply no-load power to bring the turbine rotor up tonormal operating speed and to effect sucient flow through the turbine toprevent overheating, second starting valve means connected to admitlimited quantities of steam from the steam generating means at saidinlet temperature and pressure to said lower pressure turbine section inquantities sufficient to generate no-load power and prevent overheating,and means connected to actuate said starting valve means and interceptValve means (l) to progressively open the starting valve means while thepressure regulating valve means regulates the steam bypassed to thecondenser, with the intercept valve means remaining closed; and (2) toprogressively close the starting valve means as the intercept valvemeans opens;

and (3) to maintain the starting valves completely closed and theintercept valve completely open in normal operation, with tne pressureregulating valve means bypassing no steam to the condenser.

2. For use in a compound reheat typesteam turbine Powerplant havingmeans for generating steam at a high inlet temperature and pressure, afirst turbine section connected to receive motive fluid at said highinlet conditions, at least one reheater for reheating the steamdischarge from said first turbine section, at least one lower pressureturbine section connected to receive motive fluid from said reheater,and intercept valve means controlling the ow of steam from the reheaterto the lower pressure tur-` bine, the combination of irst starting valvemeans connected to admit limited quantities of motive fluid from thesteam generating means at said inlet temperature and pressure to the rstturbine section in quantities lsucient to supply no-load power tobringthe turbine rotor up to normal operating speed and to eectsuilcient ow through the turbine to prevent overheating due to iluidfriction, second starting valve means for admitting limlted quantitiesof steam from the steam generating means at said inlet temperature andpressure to said lower pressure turbine section in quantities suflcientto generate noload power and prevent overheating due vto iluid friction,and means for actuating said starting rvalves to admit motive fluid fromVthe steam generating means simultaneously to said rst and secondturbine sections, said actuating means being connected to progressivelyclose the starting valves as the intercept valve means opens, thestarting valves being completely closed when the intercept valve meansis wide open.

No references cited,

